The present invention relates generally to braking systems and, more specifically, to brake actuators. Even more particularly, the present invention relates to a tamper-resistant spring brake actuator and a method of assembling the same.
Fluid-operated braking systems have long been used to control the movement of motor vehicles in a safe and effective manner. In particular, air brakes are commonly used on commercial vehicles such as trucks, which typically have large gross vehicle weights. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component which is instrumental in the operation of air brake systems is the brake actuator, which provides the force necessary when braking a vehicle.
In a typical double diaphragm spring brake actuator, a barrel-shaped power spring stores potential energy and exerts the large force required for braking in case of air pressure failure. Air pressure acting on a diaphragm compresses the power spring and maintains it in its brake release position. When the air is exhausted, the power spring acts on the diaphragm, typically an elastomeric diaphragm or a piston, and thereby applies the brakes in case of failure of the system air pressure.
The power spring is positioned in a spring chamber, which is typically formed by clamping an elastomeric diaphragm between a head (sometimes also known as a spring housing or spring chamber) and a flange case (sometimes known as an adaptor). The power spring is compressed within the spring chamber between the head and the diaphragm. The power spring has a high spring constant and is normally compressed to a height of less than 3 inches from an original uncompressed height of from 9 to 12 inches. The power spring, therefore, stores a substantial amount of potential energy, usually exerting a force on the head of about 2,000 to 3,000 pounds.
Unauthorized removal of the head, therefore, could be dangerous due to the large potential energy of the power spring. In addition, unauthorized replacement of the head could result in a poor diaphragm seal or other internal defects. Therefore, it is conventional to discourage unauthorized removal and replacement of the head by attaching a warning label and/or providing a securing means which is tamper-resistant or at least tamper-evident, such that a new securing means, not readily available to unauthorized personnel is required before the head can be reattached.
Various approaches have been heretofore proposed for securing the brake actuator head to the flange case to prevent improvident disassembly of the two parts. GB Patent No. 2,000,225, for example, discloses in one embodiment a brake actuator including a head secured to a flange case with a circlip. The circlip is seated in an inwardly facing groove of the flange case and extends over an annular lip of the head, such that the head can only be separated from the flange case by deforming or destroying the flange case. The circlip, however, is accessible from between the flange case and an edge of the head and, therefore, could possibly be tampered with. Moreover, the design is complex, and therefore difficult and costly to manufacture and assemble.
The ""225 patent, in another embodiment, also discloses a brake actuator including a head having a lower edge that is spun, or inelastically deformed, over a flange case, such that the head can only be separated from the flange case by deforming or destroying the lower edge of the head. Similar arrangements are also disclosed in U.S. Pat. Nos. 4,850,263, 4,960,036, 5,067,391, 5,205,205, 5,263,403, 5,311,809, 5,433,138, and 5,640,894. A disadvantage of each of these prior art references is that special deforming machinery is needed to assemble the brake actuator, making assembly both costly and difficult. Another disadvantage is that the deformed lower edge of the head can easily be undeformed by an unauthorized person attempting to disassemble the brake actuator.
U.S. Pat. No. 5,285,716 discloses a brake actuator having a head and a flange case welded together in a tamper-proof manner. As disclosed in the patent, however, it is a relatively easy task to grind away the weld to remove the head. In addition, the flange case must disadvantageously be made of steel in order to be welded to the steel head. Flange cases are normally made of aluminum.
U.S. Pat. Nos. 5,315,918 and 5,353,688 both disclose, in one embodiment, a brake actuator including a head screwed onto a flange case and secured with an insert or a pin creating an interference fit. The insert or pin may be welded in place. In another embodiment of the ""918 and ""688 patents, and in U.S. Pat. No. 4,887,513, a brake actuator is disclosed having a head which is joined to a flange case with a bayonet connection. U.S. Pat. No. 5,560,280 discloses a spring chamber head and adapter head, which include a plurality of cooperating peripheral lugs and flanges thereon. The flanges are held in registration with the lugs by the compression of the periphery of the spring brake diaphragm between the heads. A disadvantage of each of these prior art references is that the design is complex, and therefore difficult and costly to manufacture and assemble. A related disadvantage is that special and complex machinery is needed to assemble the brake actuator, making assembly even more costly and difficult.
U.S. Pat. Nos. 5,193,432, 5,775,202 and 6,131,501 each disclose spring brake actuators held together with clamp band. However, the clamp band disclosed in the ""202 patent is inelastically deformed over the flange case, the clamp band disclosed in the ""501 patent is welded to the head, and the clamp band of the ""432 patent is held in place by a screw having its head machined off after assembly. A disadvantage of each of these operations is that special machinery (i.e., deforming machinery, welding machinery, cutting machinery) is needed to assemble the brake actuator, making assembly both costly and difficult.
What is desired, therefore, is a spring brake actuator which is tamper resistant, which is simple in design as compared with known spring brake actuators, which is relatively easy and inexpensive to produce and assemble, which does not require special machinery to assemble, and which can be made of conventional materials.
Accordingly, it is an object of the present invention to provide a spring brake actuator which is tamper resistant.
Another object of the present invention is to provide a spring brake actuator having the above characteristics and which is simple in design as compared with known spring brake actuators.
A further object of the present invention is to provide a spring brake actuator having the above characteristics and which is relatively easy and inexpensive to produce and assemble.
Still another object of the present invention is to provide a spring brake actuator having the above characteristics and which does not require special machinery to assemble.
Yet a further object of the present invention is to provide a spring brake actuator having the above characteristics and which can be made of conventional materials.
These and other objects of the present invention are achieved by provision of a spring brake actuator having a flange case and a head, each of which includes a radially outwardly extending annular flange. The spring brake actuator also includes an annular retaining member comprised of two halves and having an annular peripheral groove therearound. The annular retaining member includes first and second radially inwardly protruding portions, the first radially inwardly protruding portion engaging the annular flange of the head and having an inner diameter smaller than an outer diameter of the annular flange of the head, and the second radially inwardly protruding portion engaging the annular flange of the flange case and having an inner diameter smaller than an outer diameter of the annular flange of the flange case. A retaining ring is disposed within the annular peripheral groove of the annular retaining member, and exerts compressive forces to hold the halves of the annular retaining member together to inhibit disassembly of the head and the flange case.
The retaining ring preferably comprise a continuous retaining ring, and the annular retaining member may either comprise two separate halves, or two halves joined by a hinge. In one embodiment, the annular peripheral groove around the annular retaining member is preferably located axially between the first and second radially inwardly protruding portions of the annular retaining member. In another embodiment, the annular peripheral groove around the annular retaining member is preferably located axially outside of a space between the first and second radially inwardly protruding portions of the annular retaining member, and most preferably adjacent to the first radially inwardly protruding portion of the annular retaining member.
Preferably, the spring brake actuator also includes a diaphragm clamped in a fluid tight manner between the annular flange of the head and the annular flange of the flange case. Most preferably, the diaphragm includes a periphery which comprises an annular widened portion having a thickness greater than a thickness of the remainder of the diaphragm.
In another aspect, the present invention relates to a method of assembling a spring brake actuator as described above, which method includes the steps of providing a flange case and a head, each of which includes a radially outwardly extending annular flange. The flange case and the head are compressed together until the annular flange of the head is adjacent the annular flange of the flange case. At this point, an annular retaining member comprised of two halves is disposed around the annular flange of the head and the annular flange of the flange case, the annular retaining member having first and second radially inwardly protruding portions which engage the annular flange of the head and the annular flange of the flange case such that the annular flange of the head and the annular flange of the flange case are held therebetween. Next, a retaining ring is disposed within an annular peripheral groove around the annular retaining member, which retaining ring exerts compressive forces in order to hold the halves of the annular retaining member together to inhibit disassembly of the head and the flange case.
Preferably, the retaining ring is disposed within the annular peripheral groove by elastically deforming the retaining ring, positioning the retaining ring onto the brake actuator until the retaining ring is adjacent to the annular peripheral groove, and allowing the retaining ring to deform to its original configuration, thereby exerting compressive forces to hold the annular retaining member together such that the head and the flange case are held together.
The invention and its particular features and advantages will become more apparent from the following detailed description considered with reference to the accompanying drawings.